WDM optical network with passive pass-through at each node

ABSTRACT

A communications network has a plurality of nodes interconnected by an optical transmission medium. The transmission medium is capable of a carrying a plurality of wavelengths organized into bands. A filter at each node for drops a band associated therewith and passively forwards other bands through the transmission medium. A device is provided at each node for adding a band to the transmission medium. Communication can be established directly between a pair of nodes in the network sharing a common band without the active intervention of any intervening node. This allows the network to be protocol independent. Also, the low losses incurred by the passive filters permit relatively long path lengths without optical amplification.

FIELD OF THE INVENTION

[0001] The present invention relates to a method and apparatus forestablishing communication over an optical network employing wavelengthdivision multiplexing.

BACKGROUND OF THE INVENTION

[0002] The ever-increasing demand for bandwidth has spurred the growthof high speed transport networks. Currently, the key standard for use insuch networks is SONET, which is an acronym for Synchronous OpticalNetwork. The SONET standard defines a hierarchy of optical transmissionrates over point-to-point and ring network topologies. For example, theSONET optical carrier-level 3 (OC-3) transmits at 155 Mb/s and OC-12transmits at 622 Mb/s.

[0003] SONET was developed to provide a survivable transportinfrastructure that could carry a range of different payload protocolsand payload bit rates.

[0004] Survivability is achieved in SONET using a ring topology with aSONET-defined standard protocol for coordinating traffic restoration inthe event of a failure. On a ring, there are always two diverse routesthat can be used to interconnect any two nodes on the ring. In the eventof a failure of one of those routes, spare capacity on the other routeis used to restore the traffic affected by the failure. In SONET, everynode must terminate the entire optical signal in order to be able toaccess every payload, even though typically, a node would only access asmall subset of the payloads and pass the rest of them downstream toother nodes. Termination of the entire optical signal is also requiredto give each node access to an automatic protection switching (APS)protocol that coordinates access to the spare capacity by the nodeduring failure events. Unfortunately, this requirement of SONET toterminate the entire optical signal at every node makes upgrading thecapacity of the ring a slow and costly process, because every node mustbe upgraded even though it may be that only one node requires theadditional capacity.

[0005] In order to carry a variety of payloads and payload bit rates,the SONET standard defines a payload envelope structure into which allpayloads must be mapped. This envelope is then carried within onetimeslot within the time division multiplexed SONET signal. Althoughthis provides a SONET network with the ability to carry a variety ofpayloads, a new payload cannot be transported until a mapping is definedand the interface circuit is developed and deployed. In addition, ifthere is insufficient spare capacity in the network to handle the newpayload bit rate, then the entire network may have to be upgraded. Thus,SONET networks are not responsive to the needs of today's services,which are demanding greater capacity and introducing a wide range ofprotocols.

[0006] The networks of today's telecommunications carriers typicallyconsist of an access portion that connects end-users to the carrier'snetwork, and a transport portion (sometimes called backbone or corenetwork) that provides the interconnection between the access networks.The access portion of the network is under pressure to provide a greatervariety of signal types such as asynchronous transfer mode (ATM),asynchronous digital subscriber loops (ADSL), and SONET, to handle theemerging diversity of services. These new payloads also tend to requiregreater bit rates to support the underlying services. Transport networksare under pressure to provide more capacity due to the higher bit rateservices coming out of the access networks as well as the growth in thenumber and size of the access networks reflecting the growth in thenumber of end-users.

[0007] An object of the invention is to alleviate the limitations inSONET-based networks.

SUMMARY OF THE INVENTION

[0008] According to the present invention there is provided acommunications network employing wavelength division multiplexing,comprising a plurality of nodes; an optical transmission mediuminterconnecting said nodes, said transmission medium being capable of acarrying a plurality of wavelengths organized into bands; and aninterface at each node for dropping a band associated therewith, addinga band carrying traffic for another node, and passively forwarding otherbands; whereby communication can be established directly between a pairof nodes in said network sharing a common band without the activeintervention of any intervening node.

[0009] A network in accordance with the invention is protocol and bitrate independent and is therefore more responsive than SONET to thedemands placed on the access and transport networks oftelecommunications carriers. Each payload is carried on separate opticalwavelengths and payloads are multiplexed using wavelength divisionmultiplexing techniques.

[0010] A band consists of a group of closely spaced wavelengths. A guardspace normally exists between the bands to allow for simple filtering ofa band out of the overall spectrum. For example, a band may consist offour wavelengths spaced at 1.6 nm intervals giving a total width for theband of 4×1.6=6.4 nm. With a guard band of 3.2 nm, the overall bandspacing would be 6.4+3.2=9.6 nm.

[0011] A band is associated with the connection between two nodes, suchthat if, for example, node A wishes to communicate with node C viaintervening node B, both node A and node C must access (add/drop) thesame band, say band X. Node A would transmit to node C on band X, whichwould be passively forwarded by intervening node.

[0012] The use of bands as distinct from discrete wavelengths allows thefilter specifications to be relaxed in the area of sideband roll-offslope since there are cascaded filters involved at each node. A primary(or band) filter discriminates a band of wavelengths. Furthersub-division into specific channels is done with a narrow widthfilter(s) that is sub-tended after the band filter.

[0013] The use of a multi-level filtering approach is more energyefficient than other arrangements for ring networks. This is due to thefact that the band filter is the primary filter element that is repeatedaround the ring. As nodes are added to the ring, the attenuation lossdue to the band filter element does not rise as fast as the case whereindividual wavelengths are filtered out at a node with the residual bandbeing passed on.

[0014] The interface is typically in the form of a filter whichseparates out the band to be dropped and forwards the other bands byreflection. The filter acts as a multiplexer/demultiplexer which dropsand adds the band associated with the node from the transmission medium.

[0015] The filter is preferably an interference filter with minimalloss, preferably less than 1 dB, and typically 0.5 dB. The division ofthe wavelength spectrum into bands, each associated with a node, is animportant factor in reducing the loss at the passive filter. Ifindividual wavelengths were employed, losses in the order of 3 to 6 dBcould be expected, and the maximum size of the network would be verymuch restricted.

[0016] An important advantage of the present invention is that eachwavelength essentially provides a protocol independent high speed bitpipe between a pair of nodes with minimal loss.

[0017] A node in one embodiment may also include a cross connect switchfor changing wavelengths. For example, if a path is established betweennode A and node C over band c, and between node C and node F over bandf, and no path exists between node A and node F, node A can send trafficfor node F first to node C, which drops the band c, detects that thetraffic is for node F, and passes it through the cross connect toforward the traffic in band f, which will be dropped by node F.

[0018] The optical path for the network is thus passive except for nodeswhere wavelengths are add/dropped. The system also has low overall lossin any wavelength path so that no optical amplifiers need be employed toachieve a 30 km ring circumference. The overall power loss budget isestimated at 30 dB.

[0019] In a typical maximum configuration system, approximately ⅓ of theoptical loss is in the fiber (˜9 dB) and approximately ⅓ the loss is inthe optical add/drop filters (16 band filters @0.5 db=8 dB). Theremainder of the 30 dB optical power budget is reserved for connectorloss, splices and aging of components.

[0020] According to another aspect of the invention there is provided amethod of establishing communication over an network employingwavelength division multiplexing and having a plurality of nodesinterconnected by an optical transmission medium capable of carrying aplurality of wavelengths organized into bands, the method comprising thesteps of sending traffic destined for a remote node in a band associatedwith the remote node; passively forwarding said band at any interveningnodes; and dropping said band at said remote node to extract saidtraffic destined therefor.

[0021] The invention still further provides an interface device for usein an optical network employing wavelength division multiplexing,comprising a demultiplexer for dropping a predetermined band ofwavelengths from the network at a node, means for converting opticalinput signals from said demultiplexer to electrical output signals,means for generating optical output signals from electrical inputsignals, a multiplexer for adding said optical output signals in apredetermined band to the network, said demultiplexer and multiplexerbeing arranged to forward passively optical signals in bands other thansaid band that is dropped.

[0022] In another aspect the invention provides a fiber optic wavelengthdivision multiplexed ring comprising a plurality of switching nodes,means for generating a plurality of wavelengths organized in to bands onsaid ring, and means for transmitting maintenance channel data on atleast one of said wavelengths as a pilot tone.

[0023] The maintenance channel can conveniently be injected bymodulating the bias current of the device generating the wavelengths,normally a laser.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The invention will now be described in more detail, by way ofexample only, with reference to the accompanying drawings, in which:

[0025]FIG. 1 is a block diagram showing the physical layout of awavelength division multiplexed (WDM) bi-directional ring network;

[0026]FIG. 2 is a chart of the bands of wavelengths employed in atypical system in accordance with the invention;

[0027]FIG. 3 is a block functional diagram of a network node;

[0028]FIG. 4 is a block diagram network node with an optical crossconnect switch;

[0029]FIG. 5 illustrates a ring showing the waveband connections;

[0030]FIG. 6 illustrates a ring showing protection switching;

[0031]FIG. 7 is an example of a hubbed connection pattern;

[0032]FIG. 8 is an example of a meshed connection pattern;

[0033]FIG. 9 illustrates a ring showing examples of payload signals;

[0034]FIG. 10 shows a bit rate consistency monitor;

[0035]FIG. 11 is a block diagram of a maintenance channel signal driver;

[0036]FIG. 12 is a graph showing the spectral density of eachwavelength; and

[0037]FIG. 13 is a block diagram of a distributed ATM switch.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] Referring now to FIG. 1, a WDM (Wavelength division Multiplexed)ring network generally referenced 1 consists of two counter rotatingrings 2, 3 containing a plurality of nodes 4, 5, 6, 7, 8 providinginterfaces to the rings 2, 3. It will be understood that FIG. 1 showsthe physical layout of the network. The rings 2, 3 physically consist ofoptical fibers, which are capable of carrying multiple wavelengthsgenerated by lasers in the nodes. The interconnectivity between thenodes is provided by WDM connections in a manner to be described.

[0039] Each ring may carry, for example, 16 or 32 wavelengths dividedinto eight bands, which provide the interconnectivity between the nodes.Typically there are either two or four wavelengths per band. With eightbands, there are therefore a total of 16 wavelengths per ring for twowavelengths per band or 32 wavelengths in the case of four wavelengthsper band, for example, spaced at 1.6 nm with a guard band of 3.2 nm fora total spacing of 9.6 nm per band. FIG. 2 shows a typical distributionof bands and wavelengths. Typically the maximum number of nodes iseight, assuming eight bands.

[0040] Each node 4, 5, 6, 7, 8, adds/drops a predetermined band ofwavelengths specific to that node. For example, node 6 might drop the1.52411 to 1.51948 μm band, which includes wavelengths at 1.52411,1.52256, 1.52102, and 1.51948 μm. In order to establish a path betweennode 4 and node 6, node 4 transmits to node 6 in this band on bothcounter rotating rings 2, 3. This band is passively reflected by nodes5, 7, 8 and forwarded to node 6, which drops the band and extracts thetraffic. In accordance with the principles of the invention, the bandsof wavelength thus permit direct, protocol independent connections to bemade between any nodes on the ring without the intervention of anyintermediate node. The nodes on the ring can be interconnected in anytraditional manner, for example, in star or mesh connections, byestablishing waveband connections between desired pairs of nodes.

[0041] A typical node with a wavelength conversion function will bedescribed in more detail with reference to FIG. 3.

[0042] Demultiplexers 10 and multiplexers 11 are shown connected intothe fiber optic rings 2, 3. Demultiplexers 10 drop, and multiplexers 11add, a specific band of wavelengths associated with the node. Physicallythe MUX/DEMUX 10, 11 each consist of a single high performance opticalinterference filter that transmits the selected band to be dropped/addedand passively reflects the remaining bands. The filters can be, forexample, dichroic filters, Bragg grating filters based on circulators,and Bragg grating based on fused biconic taper. A suitable filter ismade by JDS Fitel of Ottawa, Canada. Such filters offer low through-pathloss (<1 dB), and reasonable (<2 db) add/drop loss. The bands 13 notdropped by the demultiplexers 10 are passively forwarded through thenode.

[0043] In physical terms, the bands 13 of wavelengths that are notdestined for a particular node are passively reflected on to the nextnode and so on until they reach the destination node where they aredropped in the manner described.

[0044] The dropped band 12 from each ring 2, 3 is passed to a secondfine optical filter 19, which separates the dropped band into theindividual wavelengths. The subdivided wavelengths are passed toelectro-optic converters 14, which generate electrical signals from theoptical inputs. The electric signals are then passed to digital crossconnect switch, which connects them to payload interface devices 16providing access to the network. Alternatively, cross connect 15 permitsswitching between different wavelengths or bands. In the latter case,the cross connect 15 can be connected to additional MUX/DEMUX filters(not shown) provided at the same node for adding/dropping differentbands either on the same ring or a different ring.

[0045] The adding of a band works in the same way as the dropping of aband in reverse. Electrical signals are converted to optical form inelectro-optic converters 14 and passed to fine channel filters 18, whichcombine the specific band of wavelengths that it is desired to add. Theoutput 18 of these filters is passed to MUX 11 and combined with theforwarded bands 13. In physical terms, the added band(s) is/aretransmitted through the optical filter and combined with the forwardedbands 13, which are passively reflected.

[0046] The ‘optimum’ construction for a node filter is a 4, 6 or 10 portdevice having an in port, out port and 1, 2 or 4 ‘drop’ ports and 1, 2or 4 ‘add’ ports. As there are eight bands, there will be eightvarieties of the device, one per band. One such device is used whereever a band is to be add/dropped. Each port corresponds to a specificwavelength.

[0047] The filter is a highly integrated passive optical device. Thedesign and construction of the filter is such that 1 wavelength (approx.1 nm Bandwidth) is available from a ‘drop’ port and 1 wavelength(approx. 1 nm Bandwidth) is added to an ‘add’ port. By using identicalband filters at two points on the ring, 1, 2 or 4 wavelengths can beused to implement a bi-directional ‘communications’ pipe between thosepoints. These pipes are independent of any other wavelengths on thefiber ring so long as no other nodes use the same band filters. Eachwavelength used in the system in effect serves as a transparent digital“bit pipe”. Any specific formatting of data in a wavelength is to becarried out by sub-tending equipment. The system is not protocoldependent. The present invention employs bands to provide protocolindependent direct connections between nodes on a physical ring, whichin physical terms need not be adjacent.

[0048]FIG. 4 shows a similar arrangement to FIG. 3, except that theelectro-optical converters have been omitted and the cross connectswitch 115 is an optical switching device that performs opticalswitching and provides optical outputs to optical interfaces 116. Withoptical switching, wavelength conversion can be realized optically.Electro-absorption devices and/or semiconductor optical amplifiers(SOAs) may be used to perform the conversion.

[0049] Each node typically has at least one band filter, at least onelaser diode, driver and MTCE (maintenance channel modulator), at leastone PIN diode receiver, transimpedance amplifier, limit amplifier andMTCE demodulator, fine optical filters, a maintenance channel controlprocessor, with Ethernet port and node control HW, and a per wavelengthinterface to subtending equipment or test data generator (optionmodule). Optionally, a PLL data regenerator and cross-connect matrix canbe provided. A low frequency bandpass filter picks off the maintenancechannel data and it is demodulated by a PLL type FSK demodulator. Thedata stream then goes to the maintenance processor.

[0050] Each wavelength is driven by a DFB (Distributed Feedback) laseror Bragg grating reflector laser at a specific wavelength chosen tomatch the filter characteristics. The output power of the laser is inthe range of −3 dbm to a possible high of +6 dbm. Laser components mayrequire thermal stabilization (also providing a means of micro-tuning tospecific wavelengths). The laser is driven by a single chip controlcircuit that combines the monitor diode and bias requirements of thedevice. Typically these devices have a PECL differential input.

[0051]FIG. 5 shows one example of a connectivity diagram of a networkoperating in accordance with the invention. The nodes are physicallyinterconnected in a ring by counter-rotating optic fiber rings asdescribed with reference to FIG. 1. The bands of wavelengths provideddirect WDM protocol independent connections between non adjacent rings.In FIG. 5, band X connects node 5 to node 8 in both directions. Thismeans that node 5 and 8 add and drop band X, that is band X is passivelyreflected by the interference filter at node 4.

[0052] The invention also allows protection switching, with the bandsacting as direct connections between the nodes. FIG. 6 is an example ofprotection switching.

[0053] In FIG. 6, a band connects X two nodes via two diverse paths onopposite arcs 40, 41 of the ring 1. One of these arcs can be used toprovide a restoration path for all of the wavelengths in the band in theevent of a failure of the other path. In FIG. 6, a band connects nodes Aand C. The arc 40 via node B is used normally and the arc via nodes D-Zis spare. Node A and node C monitor the quality of the signals droppedfrom the band at each end of the connection. In the event of a failureof the connections via node B, nodes A and C re-establish theconnections via nodes-D-Z.

[0054] The drop nodes may use optical power measurements on eachwavelength as a quality measure. If the optical power drops below apreset threshold, then a protection switch is triggered. Thismeasurement is not dependent on the protocol or bit rate of theinformation carried on the wavelength.

[0055] Another quality measure that is protocol and bit-rate independentis a Bit Rate Consistency Monitor. The drop node counts the number ofbits received over a given unit of time (long relative to the bit timeof the lowest expected bit rate) and records the value of this count. Ifthe value varies by more than some nominal amount, it is an indicationthat the channel is carrying noise and has therefore failed.

[0056] An example of a Bit Rate Consistency Monitor is shown in FIG. 10.Incoming serial data 50 is fed to edge counter 51, which inputs an M-bitcount to register 53. A reference clock, which has a repetition rate lowrelative to the minimum serial data rate, is input to register 53 and 54to latch the count samples n and n−1 and also to the counter to reset itbetween samples. Sample n from register 53 and sample n−1 from register54 are compared in comparator 55, which generates a true/outputdepending on the consistency of the incoming bit rate.

[0057] In order to coordinate the switching of the traffic, the nodes ateach end of the connection must communicate directly For example, inFIG. 6, if a failure of the connection is observed only at node C, thennode C may have to communicate with node A to get the wavelengths thatwere being sent across the band via node B to be sent across the bandvia node D-Z. This can be accomplished using one or more of themaintenance channels that are carried on a pilot tone on eachwavelength.

[0058]FIG. 7 shows a hubbed connection pattern as a further example ofthe manner in which the nodes can be interconnected in accordance withthe principles of the present invention. In FIG. 7, node C (5) acts asthe hub from which “connections” are established to other nodes overdedicated wavebands extending between the hub node 5 and the remainingnodes.

[0059]FIG. 8 shows a meshed arrangement, where the nodes are connectedin the form of a mesh pattern. In all cases the wavebands act as bitpipes establishing protocol independent high speed connections directlybetween nodes, which may be non-adjacent.

[0060]FIG. 9 shows examples of a signal payload that may be carried by aring operating in accordance with the invention. In FIG. 9, band 30establishes a protocol independent connection between nodes 4 and 7.This connection can carry SONET OC-3 traffic and Fiber channel trafficdirectly between the nodes. The system architecture does not need toknow anything about the protocols. The band 7 merely delivers a highspeed bit stream at node 7, which can be resolved into SONET and Fiberchannel streams at the far end node.

[0061]FIG. 9 also shows Gigabit Ethernet and SONET OC-48 traffic betweensent between nodes 5 and 8. Again the ring is indifferent to theprotocols involved. The data is merely transported as a high speed bitstream over the carrier wavelength without regard to the underlyingprotocol.

[0062] It is of course possible to cascade multiple rings,interconnecting them at common nodes. However, if many rings arecascaded, dispersion effects and jitter effects on theelectrical/optical signals may accumulate. In order to compensate forthis, a regeneration stage may need to be added to the cross-connectmatrix at selected interconnect points. This re-generation device is awide range PLL (phase-locked loop) that locks onto the incoming digitaldata stream, recovers a clock and uses the clock to regenerate thedigital stream. It is not necessary for the re-generator to know thespecific format of the data in the stream, only to be able to lock ontothe digital transitions and clean up the edges with respect todispersion and jitter.

[0063] If the PLL requires setting for lock range or center frequency,this can be accommodated by maintenance channel configuration messagesthat are directed to the hardware that needs to be controlled.

[0064] A variety of external data sources can be connected to the datapath for each wavelength. This can include OC-3, OC-12, a proprietaryinterface such as Newbridge ISL (Inter Shelf Links) and possibly GigabitEthernet sources.

[0065] As mentioned above, the wavelengths carry a maintenance channel,which is driven by an FSK modulator originating directly from themaintenance channel data stream.

[0066] In a multi-node WDM ring network the opportunity exists foroverall optimization economies that can be facilitated by the individualnodes being able to communicate with each other to exchange informationthat is used to control the laser device by adjusting the fundamentaloperating parameters of the device and other optical elements that maybe used in such a network. Methods of local optimization of laserparameters have been discussed in the literature. However, this approachuses and end-to-end approach which is more complete in terms of beingable to adjust for network operating parameters. It also allowsconsideration to be given for specification tolerance reduction of othernetwork elements e.g. filter roll off centre frequency position and gainelements that may be present in such a ring configuration.

[0067] For each wavelength operating between two points in the WDM ring,there is a laser source, an add filter (or portion of an add/dropfilter), fibre transmission media a drop filter (or portion of anadd/drop filter), an optical detector and ancillary receive electronicsto route the signal to other portions of the system.

[0068] The laser source is controlled by setting a laser currentthreshold, modulation current level and operating temperature. Theoperating wavelength is adjusted (by temp control) so as to providemaximum signal energy to the detector at the far end. This procedurealigns the emitted wavelength with the combined cascade filter responseso as to minimizes the losses due to individual component tolerances. Italso has the benefit of any medium to long term wavelength variation ofthe laser from the system loss plan calculations.

[0069] The peak optical power and the extinction ratio (ER) is regulatedand controlled by special electronic circuits or by an embeddedmicrocontroller. Laser slope efficiencies at bias level and at peaklevel can be measured by varying bias current and peak current in verysmall steps respectively. Such measurement allows the laser ER and peakpower to be frequently monitored and controlled.

[0070] Wavelength stability is attained by adjusting the laser operatingwavelength (e.g. adjusting laser operating temperature) while monitoringthe received power level at the receiving node. Since the WDM filtermodules have a narrow pass band (approximately 1 nm) for each wavelengthchannel and possesses other optical characteristics, it is possible toprecisely re-align the laser operating wavelength on a regular basis.

[0071] In certain circumstances when operating the same wavelength ontwo different segments of the ring it may be necessary to set theoperating wavelengths at slightly different points so as to minimize‘beat noise’ (a coherent interference effect between optical sources).This noise factor is overcome-by having the operating wavelengthsseparated by come small amount (0.05 nm to 0.2 nm).

[0072] The other parameters of laser operation (threshold) andmodulation depth are controlled end-to-end in such a way as to optimizethe receive eye signal for a given data rate. The maintenance signalthat is superimposed on the optical wavelength gives a means ofmeasuring the error performance of the mtce channel (which is directlyproportional to the error rate of the main data channel on the opticalbeam.). By means of message feedback, the received signal conditionstate can be sent back to the laser transmitter so that correctiveactions or stabilization routines can be run. The specific controlroutines are software algorithms that run on the embedded processor thatis associated with the laser control circuitry. These control algorithmswill include both wavelength stabilization routines and received eyesignal optimization.

[0073] In a network of WDM laser sources and receivers (includingdrop/add filters) it may be necessary to add amplification to individualwavelengths or groups of wavelengths so as to obtain sufficient opticalenergy to achieve a desired bit error rate at the receiver. In a knowntopology situation a number of EDFA elements could be added to thesystem to overcome transmission losses due to fibre and filter loss. Dueto the topology of the ring and the fact that it is a ringconfiguration, fixed gain blocks such as EDFA's may be difficult tospecify and may in fact impair the performance of some wavelengths inthe system.

[0074] A solution to the problem exists in the form of a SOA elementcombined with an electrically programmable attenuator element. Thistechnology can be obtained in discrete element form or integrated onto asilicon waveguide structure. The SOA provides the pure gain required theprogrammable attenuator allows for signal level optimization on a nodeto node basis, independent of the levels required for any other node tonode level on the ring. The SOA/Attenuator combination may be applied atthe laser source (as a post-amplifier), the receiver (as apre-amplifier) or both.

[0075] In the event that it was desired to use EDFA elements as the gainblock in a fibre based ring system, the programmable attenuator could beused to optimize the receive signal level at the receiver. It is feltthat this would not provide as flexible a solution as the SOA/attenuatorsystem but would overcome the problem of individual channels gainadjustment that is required in such an amplified add/drop system.

[0076] In all cases the key to the system optimization is the abilityfor the system to communicate on the mtce channels betweensource/receiver pairs and to optimize via control algorithms theoperating level of the signal in such a way as to obtain the bestend-to-end performance and WDM network management.

[0077] The maintenance and control processor of each node is a smallcomputer board that contains processor, ram, flash memory for programand application store and several serial interfaces (one per MTCE link).The processor has an embedded DOS that is augmented with a TCP/IProuting and control module (Flexcom router and control switch). Sincethe Flexcom product is actually a Multi-tasking O/S operating inconjunction with the embedded DOS, several monitor and control functionsspecific to laser operation and maintenance are integrated into this SW.A status and reporting function is also incorporated.

[0078] By means of this switch, all nodes in a system may be controlledand monitored by a remote PC that operates a Telnet session to eachprocessor. Maintenance traffic may also be routed through a maintenanceprocessor to other nodes or subtending equipment.

[0079]FIG. 11 shows the arrangement for creating the maintenancechannel. Driver 90 for laser 91 has a bias input 92 that is modulated byFSK modulator 93 receiving at its input the maintenance channel 95. Thedata channel, which is all digital, is applied to the main input of thedriver 90.

[0080] The arrangement shown in FIG. 11 embeds a pilot tone on eachwavelength, which may be of low bit rate (<256 kbps). This pilot tone isinjected into the wavelength channel by modulation of the bias currentby the FSK modulator 93 that modulates the MTCE channel data stream intoa sequence of tones. Other carrier modulation systems such as QAM-64 orQAM-256 or OFDM may be used. The level of the pilot tone isapproximately 20 dB below the main data path. The effect of the pilottone on the BER of the main data channel, which is purely digital, isminimal since it is carried on a portion of the spectral distributioncurve well outside the portion carrying the high bit rate data (see FIG.12).

[0081] The MTCE channel modulation ensures wavelength integrity betweennodes, provides a power level estimate of wavelength link, provides nodestatus and monitoring (SNMP, RMON type messaging), distribution ofnetwork level timing synchronization, and SW & FW downloads for nodeprocessors and sub-tended equipment.

[0082] Since the MTCE channel is modulated independently from theregular ‘data pipe’ channel, the MTCE does not need to know what theformat of data in the main channel is. This is extremely important inallowing format independence of end user applications and access.

[0083] The described arrangement provides a network capable of carryingdata in the terabit/sec range over distances of up to 30 km, and morewhen cascaded rings are provided. It also allows the components of highspeed switches,.such as ATM switches, to be distributed over a campuswide network, resulting in substantial savings in trunk cards andprocessors. FIG. 13 is an example of such a distributed switch. Switchcomponents 100 are interconnected over ring 1 using nodes 110interconnected in the manner described above.

We claim:
 1. A communications network employing wavelength divisionmultiplexing comprising: a plurality of nodes; an optical transmissionmedium interconnecting said nodes, said transmission medium beingcapable of a carrying a plurality of wavelengths organized into bands;and an interface at each node for dropping a band associated therewith,adding band carrying traffic for another node, and passively forwardingother bands; whereby communication can be established directly between apair of nodes in said network sharing a common band without the activeintervention of any intervening node.
 2. A communications network asclaimed in claim 1, wherein said interface includes a filter thatpassively reflects bands other than the band(s) to be dropped at thenode.
 3. A communications network as claimed in claim 2, wherein saidfilter is an interference filter.
 4. A communications network as claimedin claim 3, wherein each node further comprises a fine filter acting asa demultiplexer for subdividing wavelengths within a band dropped bysaid filter.
 5. A communications network as claimed in claim 4, whereinsaid interface comprises photodetectors associated with said respectivesubdivided wavelengths for producing electrical signals.
 6. Acommunications network as claimed in claim 5, wherein said interfacefurther comprises lasers for generating wavelengths in said band to beadded, said lasers being directed into a multiplexer for combiningindividual wavelengths into a band.
 7. A communications network asclaimed in claim 6, wherein said interference filter further acts amultiplexer multiplexing said band to be added with bands that arepassively passed through said interface unit.
 8. A communicationsnetwork as claimed in claim 1, wherein at least some of the nodesfurther comprise a cross-connect switch for selectively connectingdesired wavelengths with a payload signal.
 9. A communications networkas claimed in claim 8, wherein said cross connect switch is anelectrical switch and electro-optical couplers are provided between saidswitch and said interface.
 10. A communications network as claimed inclaim 8, wherein said cross connect switch is an optical switch, andsaid payload signal is an optical signal.
 11. A communications networkas claimed in claim 8, wherein the wavelengths within a band haveapproximately 1.6 nm separation.
 12. A communications network as claimedin claim 11, wherein the bands are separated by approximately 9.6 nmincluding guard space.
 13. A communications network as claimed in claim11, wherein the passband of each band is approximately 6.4 nm.
 14. Acommunications network as claimed in claim 1, wherein said nodes arephysically arranged in a ring.
 15. A communications network as claimedin claim 14, wherein said ring is a bi-directional ring.
 16. Acommunications network as claimed in claim 15, wherein said nodes areconnected in a hub arrangement by said bands of wavelengths.
 17. Acommunications network as claimed in claim 15, wherein said nodes areconnected in a mesh pattern by said bands of wavelengths.
 18. Acommunications network as claimed in claim 15, wherein saidbi-directional ring is based on a single strand optical fibre.
 19. Acommunications network as claimed in claim 1, wherein said nodes arephysically arranged in an open-loop or a point-to-point manner.
 20. Acommunications network as claimed in claim 19, wherein said network isbi-directional.
 21. A communications network as claimed in claim 19,wherein said network is based on a single strand of optical fibre.
 22. Amethod of establishing communication over an optical network employingwavelength division multiplexing and having a plurality of nodesinterconnected by an optical transmission medium capable of carrying aplurality of wavelengths organized into bands, comprising the steps of:sending traffic destined for a remote node in a band associated with theremote node; passively forwarding said band at any intervening nodes;and dropping said band at said remote node to extract said trafficdestined therefor.
 23. A method as claimed in claim 22, wherein saidnodes are physically arranged in the form of a bi-directional ring. 24.A method as claimed in claim 22, further comprising the step of adding aband of wavelengths at a node for transmission over said network to aremote node dropping that band.
 25. A method as claimed in claim 24,further comprising-the step of performing wavelength switching at a nodeto transfer data between bands.
 26. An interface device for use in anoptical network employing wavelength division multiplexing, comprising ademultiplexer for dropping a predetermined waveband from the network ata node, means for converting optical input signals from saiddemultiplexer to electrical output signals, means for generating opticaloutput signals from electrical input signals, a multiplexer for adding asaid optical output signals in a predetermined waveband to the network,said demultiplexer and multiplexer being arranged to forward passivelyoptical signals in bands other than said band that is dropped.
 27. Aninterface device as claimed in claim 25, wherein said multiplexer anddemultiplexer comprise an optical filter that extracts said droppedwaveband and reflects said other wavebands.
 28. An interface device asclaimed in claim 26, wherein said optical filter is an interferencefilter.
 29. A fiber optic wavelength division multiplexed ringcomprising a plurality of switching nodes, means for generating aplurality of wavelengths organized in to bands on said ring, and meansfor transmitting maintenance channel data on at least one of saidwavelengths as a pilot tone.
 30. A fiber optic wavelength divisionmultiplexed ring as claimed in claim 29, wherein said transmitting meansfor transmitting maintenance channel data modulates the bias current ofsaid means for generating a plurality of wavelengths.
 31. A fiber opticwavelength division multiplexed ring as claimed in claim 30, whereinsaid transmitting means FSK modulates the maintenance channel data ontosaid at least one wavelength.
 32. A fiber optic wavelength divisionmultiplexed ring as claimed in claim 31, wherein said transmitting meansQAM modulates the maintenance channel data onto said at least onewavelength.
 33. A fiber optic wavelength division multiplexed ring asclaimed in claim 30, wherein said transmitting means OFDM modulates themaintenance channel data onto said at least one wavelength.
 34. A fiberoptic wavelength division multiplexed ring as claimed in claim 29,wherein said maintenance channel has a modulation depth such that itmatches the bit error rate (BER) or waterfall curve performance of thedata channel so that the system can determine the performance of thedata channel by monitoring the maintenance channel data and withoutlooking at the maintenance channel payload.
 35. A distributed-high speedpacket switch comprising a plurality of switching components distributedover a geographic area; a fiber optic wavelength division multiplexedring interconnecting said switching components, said fiber optic ringcarrying a plurality of wavelengths organized into bands; and means foradding/dropping a band at each switching component associated therewith,said adding/dropping means passively forwarding other bands, and pairsof said switching components forming part of said switch directlycommunicating on wavelengths in bands associated therewith.
 36. Adistributed high speed packet switch as claimed in claim 35, whereinsaid switch is an ATM switch.
 37. In a communications network employingwavelength division multiplexing comprising a plurality of nodes, and anoptical transmission medium interconnecting said nodes via individuallinks, a method of optimizing the individual link performance (receivedsignal quality, power level, bit error rate) between any communicatingnodes in the network comprising sending data over maintenance channellinks.
 38. In a communications network employing wavelength divisionmultiplexing comprising a plurality of nodes, and an opticaltransmission medium interconnecting said nodes via individual links, amethod of determining whether a data channel is carrying valid trafficor noise comprising verifying the consistency of the bit rate of thedata channel.
 39. In a communications network employing wavelengthdivision multiplexing comprising a plurality of nodes, and an opticaltransmission medium interconnecting said nodes via individual links, amethod of determining whether pulse retiming is required and controllinga frequency agile clock recovery and jitter filter circuit comprisingmeasuring the bit rate of a data channel.